I just did this with an AR 9mm carbine at work. I counted the number of turns after marking one notch in the sight with liquid paper. It was a crude method but the rifle was on paper at 50 yards. A couple of more clicks to bring it down and we were good. You will want to verify your point of impact at the range anyway so as long as your close enough to put the rounds on paper I wouldn't worry about being exact when you swap sight posts.